TRIP (Telephony Routing over IP) protocol defined in IETF RFC3219 provides a method of exchanging route information in an operator domain (such as ITAD, Internet Telephony Administrative Domain), i.e. among LSs (Location Server). However, TRIP does not define how to select a specific MGW (Media Gateway) to set up a connection for a call requested by a phone, and there is no routing scheme defined in LSs either.
Draft-ietf-iptel-tgrp defines TGREP (Telephony Gateway Registration Protocol). TGREP allows PSTN (Public Switched Telephone Network) gateways or switches to inform a signaling server, such as BGCF (Breakout Gateway Control Function) or MGCF (Media Gateway Control Function), of the signaling server's routes to PSTN. Advertisement information includes fairly dynamic information, such as remaining capacity in a particular trunk. However, TGREP only specifies how to collect the dynamic information and does not specify how the signaling server selects an MGW based on the collected information.
3GPP and TISPAN (Telecommunications and Internet converged Services and Protocols for Advanced Networking) specify that there must be a routing function in BGCF, but they do not specify routing schemes or routing algorithms.
Currently, in a network based on H.323 or ordinary Internet VoIP, a signaling server selects an MGW by applying a polling method or a random selection method.
MGW is a breakout point where a session, requested by a UE (User Equipment) to be set up, is transferred from a packet switching domain to a circuit switching domain. Performance and reliability of an MGW plays a very important role in providing a successful session. If an MGW is selected by applying a polling method or a random selection method, during heavy traffic period, an MGW having load heavier than a certain level will experience a sudden degradation of performance or even a breakdown, because the MGW's load is too heavy. Therefore, service quality of the network will decrease sharply, which may lead sessions to be terminated or not be able to be set up.